In the common converter steelmaking method, dephosphorization refining and decarburization refining of molten iron are simultaneously performed in a single converter to terminate the steelmaking operation. In a recent steelmaking method, however, required quality of steel products has become higher, and a continuous casting or a secondary refining of molten steel in a vacuum degassing furnace, a ladle refining furnace or the like has become widely used. Therefore, a tapping temperature in the converter is raised to decrease dephosphorization ability in the converter.
So, there has been developed a molten iron pretreatment method in which molten iron to be charged in a converter is preliminarily subjected to dephosphorization to remove phosphorous in the molten iron and then charged into the converter. Since removal of phosphorus is efficient in dephosphorization at a molten iron stage of a low temperature level, it is advantageous to perform a method of preliminarily removing phosphorus at a molten iron pretreatment step. For example, as a refining method for molten iron pretreatment, there are a torpedo car system, a ladle system, a converter type system different from a converter for decarburization refining and so on. They are methods of co-using addition of quicklime, iron oxide or the like from above or by an injection method and agitation by blowing of stirring gas or blowing of oxygen gas. In the dephosphorization refining being oxidation refining, since silicon has a stronger affinity for oxygen as compared to phosphorus, silicon in the molten iron is previously removed by oxidation.
Heretofore, the efficiency of the converter and improvement in productivity are attained by performing desiliconization and dephosphorization refining at the molten iron stage and mainly performing decarburization refining in the converter. In such a conventional technique, however, a relatively low phosphorus level can be achieved, but potential heat of the molten iron cannot be utilized effectively due to a long treating time, a large heat loss during treatment, a long time required for supplying to the converter, an inevitable temperature decrease in the tapping of molten iron or recharging into another converter after the treatment even in the use of two converters and so on. Also, there are problems that when the molten iron is preliminarily treated for dephosphorization, a thermal margin in the steelmaking step is decreased and flexibility of materials used is lost and recycling use amount of iron scraps charged into the converter is limited.
Therefore, there has been proposed a refining method in which the dephosphorization refining and decarburization refining of the molten iron are performed simultaneously in the single converter. For example, JP-A-2006-274349 proposes a method wherein calcium oxide containing powder is blown onto a melt surface together with oxygen gas to refine in an oxygen top blowing converter, while agitation is conducted by blowing gas from a nozzle arranged under the melt surface, during which ladle slag including aluminum oxide or a composition including aluminum oxide are added into the furnace to realize reliable molten slag formation through blowing of quicklime powder into the converter to thereby perform accurate refining to decrease an amount of slag generation.
However, the technique disclosed in JP '349 has a problem that if the blowing control is not adequate, a part of quicklime blown into the converter does not stay in the converter due to scattering and efficiency is deteriorated.
In recent steelmaking techniques, it is essential to decrease the discharge amount of slag generated during steelmaking in correspondence with environmental impact as typified by global warming. To decrease the slag discharge amount in the dephosphorization of molten iron, it is necessary to decrease a charging amount of a dephosphorizing agent forming a slag served as the dephosphorization refining agent (hereinafter referred to as “slag for dephosphorization refining”). The essential component of the dephosphorizing agent in the dephosphorization refining of molten iron is lime (CaO). Therefore, to decrease the slag discharge amount, it is required to use a technique of maintaining a necessary dephosphorization amount while decreasing an amount of lime used, i.e., a technique of efficiently performing the dephosphorization refining at a small amount of lime used.
It could, therefore, be helpful to provide a converter steelmaking method capable of efficiently performing dephosphorization refining when molten steel is produced by simultaneously performing decarburization refining and dephosphorization refining of molten iron in a converter.